Physicians have always asked certain questions: What is the diagnosis? What is the cause? What is the remedy? How do we know? These questions are the constant factors throughout medical history. The answers constitute the variables, changing from one century to another. The questions, exceedingly complex, involve such philosophic topics as the nature of disease; classification of diseases; signs and symptoms, including the theory of signs; the nature and process of diagnosis; the nature of causation in reference to diseases; the different kinds of evidence and their validity; the "scientific method". There are technological and scientific aspects, such as the development of diagnostic techniques and alleged causal agents, both "internal" and "external". The philosophic attitudes and the scientific achievements relate to sociological factors. By studying all these in a historical dimension, relative to medicine, we can better understand the dynamics of medical changes. In my analysis I will restrict myself to the period from the 16th to the mid-20th centuries and will attend especially to tuberculosis and diabetes as examples of infectious and non-infectious diseases, respectively.